The purpose of this clincal trial is to test the hypothesis that the mitral valve apparatus (leaflet, chordae tendineae and papillary muscles), if left intact at the time of valve replacement, will prevent the nearly uniform postreperfusion left ventricular dilation and low cardiac output associated with surgical treatment of mitral insufficiency of long duration. The specific aims are: (1) measure LV hemodynamics and dimensions with ultrasound prior to and after cardiopulmonary bypass in the OR: 2) replace the mitral valve in patients with pure mitral regurgitation with and without resection of the entire mitral valve apparatus; (3) determine immediate postoperative hemodynamic characteristics of each group; (4) analyze 6 month and 3 year results in terms of exercise capacity, LV dimensions, and hemodynamic criteria. The rationale of this investigation is the hope that if the immediate postbypass course of patients receiving mitral valve replacement for long standing mitral insufficiency can be altered by the mechanism of preventing left ventricular dilation by maintaining the innate physical structures of the left heart, operative mortality and support measures may be reduced and long-term benefits may accrue. The results in five patients have been excellent. Two patients have had bioprostheses and 3 patients have had St. Jude prostheses. Matched paired patients having mitral valve replacement during the same interval have had subtle changes of left ventricular dysfunction in the postoperative interval which appear typical and different than the experimental group. Six month data will be obtained soon.